1. Field of the Invention
The present invention relates to a silver halide photographic emulsion and a silver halide photographic light-sensitive material having an excellence in the saving of a silver and an improved reciprocity law failure, with a high sensitivity, an improved graininess, a high color density and a hard gradation.
2. Description of the Related Art
The basic photographic properties that silver halide emulsions are required to have are a high sensitivity, a low fog, and a fine graininess.
various studies have been made to increase the sensitivity of an emulsion. For example, as improvements in grain formation, JP-A-2-943 ("JP-A" means Published Unexamined Japanese Patent Application), JP-A-1-105234, and JP-A-63-285534 disclose methods of improving the halogen structures of silver halides by selecting the conditions of grain formation. As improvements in chemical sensitization, in addition to those in normal gold-sulfur sensitization, methods of improving reduction sensitization are described in, e.g., JP-A-2-222939, JP-A-3-168632, and JP-A-62-291635, and improvements in selenium sensitization are described in, e.g., JP-A-59-185330 and JP-A-3-4221.
Also, attempts to increase sensitivity by performing improvement on the surfaces of silver halide grains have been made.
EP0019917 discloses a method of depositing a silver halide containing less than 10 mol % of iodide on silver halide grains containing 15 to 40 mol % of iodide by epitaxial growth. U.S. Pat. No. 3,782,960 discloses a method of increasing sensitivity by sensitizing silver chlorobromide grains obtained by halide conversion by using 0.01 to 25 mol % of iodide, approximately 0.001 to 1.0 mol % of gold, and an effective amount of silver thiocyanate. In addition, U.S. Pat. No. 4,471,050 discloses a method of epitaxially growing silver thiocyanate or silver cyanate limitedly to the corners or the edges of host grains. In the epitaxial grain obtained by this method, a silver salt having a composition different from that of the surface of a host grain is grown as an epitaxy such that the junction portion is limited to the corner or the edge of the grain. When observed in a direction perpendicular to the major plane of the host grain, this epitaxy extends outside the major plane and is junctioned across a plurality of planes. An epitaxy of this type is low in stability and deforms when left to stand in the form of a solution for several hours. Therefore, such a technique cannot be put into practical use.
In addition, JP-A-62-124552 or JP-A-1-113745 discloses a method of manufacturing ruffle grains having a large number of recesses on their surfaces. These patent applications described that the effect is an improvement in colour sensitization rate due to an increase in surface area.
Furthermore, JP-A-1-273033 discloses a method of junctioning an epitaxy with a low iodide content on the major plane of a host grain containing 5 to 45 mol % of silver iodide. This method can form an intraface epitaxy junctioned to a single major plane. However, the position of this intrafacial epitaxy is not limited in the neighborhood of the corners of the grain, and the host grain has a high iodide content. In addition, since the color density is low, only a soft gradation can be obtained. Hence, the technique is lacking in versatility.
Although various methods have been attempted to increase the sensitivity of a silver halide emulsion as described above, the techniques to increase the sensitivity are still deficient in order to further improve the image quality of a silver halide photographic light-sensitive material. In addition, a strong demand has arisen for a technique of developing an emulsion that can achieve a high color density even with a small amount of silver without impairing the image quality.
Recently, the rate of use of strobe light in photographing using a silver halide photographic light-sensitive material, has been raised, and therefore, there is a demand for an improvement of image quality not only in the case of a regular exposure time of about 1/100 sec, but also in the case of a high illumination intensity exposure in even a shorter exposure time.
In the case where an object having a very low illumination intensity is photographed such as in astronomical photography, the photographing is performed at an exposure time as long as several seconds or more. Therefore, there is proposed a type of photographing material which has a small low illumination intensity failure.
As a technique for improving the reciprocity law for high and low illumination intensity, addition of an iridium compound during the formation of grains, during a chemical sensitization, or after a chemical sensitization is known. However, with such a technique, the improvement of the reciprocity law failure is not sufficient, and the sensitivity in the case of an exposure time of about 1/100 second is lowered.